The gastro-oesophageal junction is a specialised segment of the gut designed to prevent reflux of gastric contents into the oesophagus. This task is fulfilled by two structures, i.e. the lower oesophageal sphincter and the crural diaphragm, which generate a high pressure zone. Especially during low pressure at the junction, as in case of long-lasting transient lower oesophageal sphincter relaxations, reflux can occur but mainly if a positive pressure gradient exists between stomach and the oesphagogastric junction. Although patients with gastro-oesophageal reflux disease have increased oesophageal acid exposure compared to controls, the number of transient relaxations is not increased compared to healthy controls. Instead, the risk to have acid reflux is at least doubled in patients, especially in those with a hiatal hernia, most likely as a result of the supradiaphragmatic position of the acid pocket. In hiatal hernia patients, the acid pocket is indeed often trapped in the hernia above the diaphragm. Which factors exactly determine the physical composition (liquid or gas) and the proximal extent of the refluxate however requires further research.